The mean excitation energy < I > is a fundamental quantity in radiation physics, concerning energy deposition in matter and absorbed dose analytical estimations for charged particles. The stopping of swift ions in different materials strongly depends on this parameter among others. This work intends to fill in part, an empty hole in the theory of stopping power: the need of analitically and theoretically assess the < I >-value for materials. The definition of the mean excitation energy using the dielectric response function is analytically integrable if the inelastic cross section parameters are known. Some dielectric models were studied, aimed at calculating the < I >-value for liquid water by theoretical means, reaching the conclusion that a decay of the order of omega(-2) in frequency (energy) is needed as weak condition of the optical energy-loss function for the integrals to converge. Afterwards, the first four discrete excitation levels and the diffuse bands for water are treated in a fully analytical scheme, and further compared with numerical results, providing the contribution of these levels to < I >, with the aim of testing the proposed analytical model.